Meaning
Synthetic thermoplastic polymers produced by the polymerization of vinyl chloride monomer are widely utilized in pipes, cable insulation, medical tubing, and rigid profiles. Processing polyvinyl chloride requires careful control of temperature because the resin is highly sensitive to thermal degradation. This polymer can be formulated in both rigid and flexible grades, making it versatile across multiple industrial applications.
Processing Characteristic
Melt temperature must be kept within a narrow processing window to prevent the release of corrosive hydrochloric acid gas. For polyvinyl chloride, the use of specialized screw designs with low compression ratios and chrome-plated or stainless steel bimetallic barrels is necessary to resist corrosion. This prevents the degradation of the equipment and maintains the aesthetic quality of the molded parts.
Formulation Variable
Compounders add thermal stabilizers, plasticizers, lubricants, and impact modifiers to the base resin to make it processable. When molding polyvinyl chloride, the concentration of plasticizer determines whether the resulting part will be highly flexible or rigid. The presence of these additives also affects the melt viscosity, necessitating precise control of injection speeds and pressures.
Manufacturing Risk
Degradation occurs rapidly if the polymer melt is allowed to stagnate in the barrel or runner system during production interruptions. If polyvinyl chloride degrades, it creates black specks in the product and releases acidic fumes that can etch the highly polished surfaces of the mold cavities. To minimize this risk, molders must run purging compounds through the machine immediately before shutting down the injection molding line.
This maintenance action removes all traces of the heat-sensitive resin from the heated cylinder, preventing machine damage and reducing scrap on the next startup.
